1. Field of the Invention
This invention is embodied in a novel process for preparing a composite material suitable for use as a separator in alkaline storage batteries. The composite materials prepared in accordance with this invention are comprised of fragile inorganic cloths reinforced with fluoroplastics. Separators intended to be used in the fabrication of alkaline storage batteries for spacecraft applications, e.g. batteries such as nickel-cadmium and nickel-hydrogen batteries, must meet conditions quite unlike these for separators normally used in terrestrial alkaline storage batteries. For example, alkaline storage batteries intended for spacecraft applications must be long lived and are normally sealed cells operating in an electrolyte starved condition for optimized gas recombination. As such they must be gas permeable, chemically resistant to alkaline potassium hydroxide solutions, and wettable by said alkaline potassium hydroxide solutions. In addition, these separators must be mechanically strong in order to withstand mechanical stress incurred during battery manufacturing process and also during spacecraft operations. Preferably these cells should also be light in weight.
2. Description of the Prior Art
There are many approaches to the solution of the problem of preparing composite separators for use in alkaline storage cells. Typical examples of these approaches are disclosed in the teachings of H. Langer et al. in U.S. Pat. No. 3,749,604 for Heat Resistant Substrates and Batteries Separators made therefrom; the teachings of Frank C. Arrance in U.S. Pat. No. 3,861,963 for Battery Separator Construction; the teachings of Nigel I. Palmer in U.S. Pat. No. 3,933,525 for Battery Separator Manufacturing Process; the teachings of Joseph A. Cogliano in U.S. Pat. No. 4,110,143 for Process for Making a Wettable Polypropylene Battery Separator; and the teachings of Phillip Bersting in U.S. Pat. No. 4,122,133, for Material for an Alkaline Cell Separator and Process of Making. Many of the above mentioned teachings have yielded separators which meet with a measure of success. However, this art has not totally satisfied the requirements for alkaline storage cells to be used in spacecraft applications. Generally, these separators of the prior art are either non-wettable, thermally unstable or gas impermeable.
As far as it is known to Applicant, the closest prior art to the present invention is that disclosed in applicant's U.S. Pat. No. 4,233,347 identified above. In this above-mentioned patent, Applicant and his coinventors described a process for reinforcing structurally fragile inorganic fabrics including the materials that are reinforced by the process described below. The prior art process disclosed in U.S. Pat. No. 4,233,347 differs from that of the present invention in that the prior art process of Ser. No. 948,119 describes the use of a polymer solvent solution and the precipitation of the polymer from that solution onto the surface of the fabric to be reinforced. While this latter process has been generally successful insofar as its objectives were concerned, it is not totally satisfactory in that the polymer materials used to reinforce the inorganic fabrics tend to be either not sufficiently stable in the presence of potassium hydroxide or not as thermally stable as desired.
Conventional processing methods are unsuitable for forming composites from inorganic fabrics and fluoroplastics where gas permeability and wettability characteristics are desired in the finished product. For example, attempts to form a molded composite from these materials have yielded a non-wettable gas impermeable material. Therefore, there is a need for a composite material which exhibits all of the above-stated desirable operating characteristics and useful as separators in spacecraft power systems.